This invention relates to cladding pumped glass fiber lasers and, more particularly, to an arrangement for a fiber laser and/or amplifier for getting high gain at a short length.
Although it is known that double cladding technique is effective in construction of high power fiber lasers, so far the technique has generally been considered ineffective for pumping three-level fiber lasers such as erbium amplifiers due to the high ground-state absorption. One solution for high power pumping is to use the single mode output from other fiber lasers such as disclosed by Grubb in U.S. Pat. No. 5,530,710 and by Ball et al in U.S. Pat. No. 6,031,849. However, this approach of using fiber lasers to pump erbium amplifiers is complicated and ineffective. The cost is also high because diode laser is used to make a fiber laser that is, in turn, used for EDFA pumping. A better solution is to make double cladding fibers that have more efficient structure.
Double cladding fiber relies on a multi-mode cladding for accepting and transferring the pumping energy to the fiber core along the length of the fiber. U.S. Pat. No. 4,815,079 assigned to Polaroid Corporation, improves the efficiency for pumping beams entering the core from the inner cladding by placing the core off-center within a circularly symmetric structure or by using a high cladding/core area ratio rectangular pump cladding enclosing a centrally-located core. The cross-sectional area of the multi-mode cladding has a first path across the area which passes substantially through the geometric center of whose length is substantially different from the length of a second path across the area which passes substantially through the geometric center, wherein second path is substantially perpendicular to the first path.
U.S. Pat. No. 5,533,163, assigned also to Polaroid Corporation disclosed the cross-sectional shape of the inner cladding in the form of non-rectangular, convex polygons. The design objective is uniform distribution of pump energy across the fiber cross section, assuming effective transfer. The disclosed polygons have the property that, if a plurality of the same polygons are used to tile a plane, all of the polygons will fit into the tiling such that no spacing will be present between adjacent polygons.
U.S. Pat. No. 5,966,491, assigned to Lucent Technologies, improves the property of an otherwise circular double cladding fiber by introducing longitudinal, parallel troughs for the symmetric inner cladding.
U.S. Pat. No. 6,157,763, assigned to SDL provides a double cladding fiber with an inner cladding that is non-circular. The cross-sectional shape is such that two perpendicular distances across the shape, each of which passes through a geometric center of a core of the fiber, are equal for all angular positions, just as for a circular double cladding fiber.
In these improvements of double cladding fiber, the radiation localized in the cladding due to the local mode can not pass through the single mode inner core, causing low efficiency of the fiber, and leading to the necessity of using long length of fiber in device construction. In practice, the required length of the commercial double cladding fibers with above-mentioned Polaroid or Lucent structures is over 50 meters in the construction of a fiber laser. One of the consequences of the long length is low tolerance to the attenuation coefficient, and the fibers must be made with the expensive deposition process (MCVD) so that the transmission loss can be minimized. These large cladding dimensions also led to the conclusion that double cladding is not effective for pumping three-level fiber lasers such as erbium amplifiers. The low efficiency of the cladding structure also limits the use of many useful glass materials for making fiber lasers, because poor performance (such as high loss due to absorption) will be obtained. One example is shown by the unsatisfactory performance of a double cladding laser fiber with doped phosphate core and conventional cladding design similar to Polaroid design due to high loss and low efficiency (J. D. Minolly, et al IEEE Photonics Technology Letters, v.5. 1993. 301; R. Wu, Opto Southwest 2000).
Thus, there is a need for better double cladding structures so that more available materials can be used effectively in double cladding fiber lasers. Better fiber structures could lead to easy fabrication and low cost. This is important for low cost, high power EDFAs and other fiber lasers. Better structure will also enable the construction of other amplifiers that could operate within the window used for optical communication. An approach that would allow an increase in the active core diameter and/or decrease in the inner cladding cross section would represent a significant advance in this technology.
Accordingly, it is the principal object of the present invention to provide efficient laser fibers having a novel cladding structure and arrangement.
It is another object of the present invention to use the novel cladding structure for making high power fiber laser or erbium doped fiber amplifiers with high gain using a short fiber. High pumping efficiency and low cost can be achieved.
It is yet another object of the present invention to use the invention in making other fiber laser or rare earth doped fiber amplifiers operating within the interested wavelength window for optical communication.
In accordance with the present invention, rare-earth doped glass, selected from silica, silicate, fluorozirconate, phosphate, fluroberyllate, fluroaluminate, flurophosphate, borate, germanate, tellurite, borosilicate, phosphosilicate, germanosilicate, lead germanate, or chalcogenide, is used as the inner core for double cladding glass fiber with high efficiency inner cladding.
In accordance with the present invention, the geometry of the inner cladding of a double cladding fiber is designed to avoid local mode so that the double cladding fiber can be used for fiber laser and amplifier to provide high gain at a short fiber length.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a preferred double cladding fiber comprises an inner core surrounded with an inner cladding with a cross-sectional shape that is unstable to local-modes and an outer cladding disposed adjacent the inner cladding. The inner core is doped with active species such as rare earth ion. The characteristics of the unstable cladding is that the clad cross section is encircled by several arcs, and any opposite arcs (mirror pairs) satisfy unstable cavity conditions:
(1xe2x88x92L/r1)(1xe2x88x92L/r2) less than 0,
or
(1xe2x88x92L/r1)(1xe2x88x92L/r2) greater than 1;
where r1 and r2 are the arc radii of the inner cladding surfaces that are substantially opposite to each other (mirror pair), with the radius of a concave mirror being positive and that of a convex mirror being negative, L is the distance between two mirrors. The cyclic stability of beams in inner cladding can be further destroyed by shifting the centers of the arcs. Said inner cladding, generally called unstable inner cladding below, may be square-like, rectangular-like or polygon-like. A double cladding fiber with such inner cladding is generally called unstable double cladding fiber in the description below.
Also to achieve the foregoing objects, the area ratio of the inner cladding (no matter what shape) to the inner core of a double cladding fiber is preferably smaller than 200, more preferably smaller than 50, and further preferably smaller than 10 so that local modes can be minimized.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a preferred fiber laser or fiber amplifier may comprise a pumping source and an unstable double cladding fiber.
In accordance with the present invention, the diode laser becomes effective for pumping three-level double cladding fiber lasers when an unstable double cladding fiber is used. Thus, this technique can be used to directly pump an important class of devices that includes erbium fiber amplifiers and other three-level fiber lasers.
According to the present invention, a preferred laser amplifier (such as a Raman amplifier) for optical communication may comprise an input fiber for propagating optical signal, an output fiber for propagating amplified optical signal, an unstable double cladding fiber interposed in the path between said input and output fibers for amplifying said optical signal propagated between said input and output fibers, a diode laser array pumping source with coupling optics for injection of pumping laser beam into said unstable double cladding fiber amplifier.
According to yet another aspect of the present invention, a preferred fiber laser may comprise an unstable double cladding fiber, a diode laser array pumping source with coupling optics for injection of pumping laser beam into said unstable double cladding fiber, and having reflector at both end of the unstable double cladding fiber.
Additional objects, new features and advantages of the present invention will be set forth in part in the following description. Further scope of applicability of the present invention will become apparent from the detail description of the invention provided hereinafter. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating preferred embodiment of present invention, are provided for illustration purposes only, because various changes and modifications within the scope and spirit of the present invention will become apparent to those of ordinary skill in the art from the detail description of the invention that follows.
This invention provides a novel inner fiber cladding structure that avoid local mode and that allows the pumping beam enter the fiber core of a double cladding fiber with high efficiency, which allows highly doped glass be used as the fiber core eliminating the need of the expensive MCVD process and enables the fabrication of three level double cladding pumped erbium doped amplifiers.